The use of differential absorption LADAR (DIAL) for trace substance detection because commonplace in the late 1960s, after the invention of the laser (1960) made narrow line width, coherent sources feasible at high powers. However, the application of this technique has been confined to short-range atmospheric paths due to concentration on use of backscatter signals, which suffer from round-trip absorption and 1/r.sup.2 losses due to isotropic scattering/absorption. Use in hostile or remote locations precluded separately located receivers/detectors. The advent of a variety of remote and/or inexpensive platforms (e.g., satellite sensors, unmanned aerial vehicles [UAVs] and other airborne platforms aerostats/balloons/aircraft) is a critical factor in the development and feasibility of the systems defined herein.
Current DIAL systems consist of a transmitter and detector system with shared optics, to insure that the outgoing and return paths are identical, and rely on a short, dual frequency pulse of radiation to backscatter from a region of the atmosphere (whose location depends on the delay time of the return pulse). The backscattered radiation will indicate detection of a substance of interest if the ratio of the absorption line center frequency to the wing frequency differs statistically from unity. The ability to detect the signal will depend on the amount of radiation backscattered (which varies as 1/r.sup.2, the atmospheric absorption and the detector sensitivity and noise characteristics).